We have continued our studies of polyamines in Saccharomyces cerevisiae, utilizing biochemical and genetic methods. We have emphasized the study of the gene for S-adenosylmethionine decarboxylase, an essential enzyme in polyamine biosynthesis, in Saccharomyces cerevisiae. This protein is also important since it belongs to a small group of enzymes that have a unique requirement for activity, namely, a covalently-linked pyruvate. We have prepared a 3200-bp fragment of yeast DNA that can be maintained stably in a shuttle yeast vector. This fragment of DNA contains the complete coding sequence for the enzyme and is expressed in yeast. We have evidence for the synthesis of a longer proenzyme that is cleaved to form an active pyruvate-containing subunit. We are sequencing the gene using the dideoxy technique in order to study the mechanism of formation of the essential pyruvate that is covalently linked to the protein. We have shown that a cell division cycle defect occurs in amine-deficient cells, with the accumulation of budded forms. The more complete the amine deficiency, the more nearly equal the size of the mother cell and the bud. Thus we have shown that polyamine deficiency is associated with arrest of the cycle in a late period, just before cytokinesis. Ornithine decarboxylase, the enzyme that synthesizes putrescine in S. cerevisiae, has now been purified (75% pure) as an 86,000 molecular weight polypeptide under conditions that minimize proteolysis. We find that the enzyme is synthesized as an 86,000 molecular weight polypeptide that is easily and specifically converted to the 68,000 molecular weight form by proteolysis.